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NEUROSCIENCE
Mass of the postsynaptic density and enumeration of three key molecules

Xiaobing Chen ^*, Lucia Vinade ^*, , Richard D. Leapman , Jennifer D. Petersen ^*, Terunaga Nakagawa , Terry M. Phillips , Morgan Sheng , and Thomas S. Reese ^*, ¶

^*Laboratory of Neurobiology, National Institute of Neurological Disorders and Stroke, National Institutes of Health, 36 Convent Drive, MSC 4062, Bethesda, MD 20892; Departamento de Bioquímica, Universidade Federal do Rio Grande do Sul/Foundation of Support to the Research of the State of the Rio Grande do Sul, 90035-003 Porto Alegre, Rio Grande do Sul, Brazil; Division of Bioengineering and Physical Science, Office of Research Services, Office of the Director, National Institutes of Health, 9000 Rockville Pike, Bethesda, MD 20892; and Picower Center for Learning and Memory, Howard Hughes Medical Institute, Massachusetts Institute of Technology, E18-215, 77 Massachusetts Avenue, Cambridge, MA 02139

Contributed by Thomas S. Reese, June 25, 2005

The total molecular mass of individual postsynaptic densities (PSDs) isolated from rat forebrain was measured by scanning transmission EM. PSDs had a mean diameter of 360 nm and molecular mass of 1.10 ± 0.36 GDa. Because the mass represents the sum of the molecular masses of all of the molecules comprising a PSD, it becomes possible to derive the number of copies of each protein, once its relative mass contribution is known. Mass contributions of PSD-95, synapse-associated protein (SAP)97, and -Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) were determined by quantitative gel electrophoresis of PSD fractions. The number of PSD-95 molecules per average PSD, contributing 2.3% of the mass of the PSD, was calculated to be 300, whereas the number of SAP97 molecules, contributing 0.9% of the mass of the PSD, was 90. The -CaMKII holoenzymes, which contribute 6% of the mass when brains are homogenized within 2 min of interrupting blood flow, have 80 holoenzymes associated with a typical PSD. When blood flow is interrupted 15 min before homogenization, the average mass of PSDs increases by 40%. The additional -CaMKII associated with PSDs accounts for up to 20% of this mass increase, representing the addition of 100–200 -CaMKII holoenzymes.

mass measurements | PSD-95 | SAP97 | CaMKII

Freely available online through the PNAS open access option.

Abbreviations: AMPAR, -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor; CaMKII, Ca²⁺/calmodulin-dependent protein kinase II; LTP, long-term potentiation; NMDAR, N-methyl-D-aspartate receptor; PSD, postsynaptic density; SAP, synapse-associated protein; STEM, scanning transmission EM; TEM, transmission EM; TMV, tobacco mosaic virus.

^¶ To whom correspondence should be addressed. E-mail: treese{at}mbl.edu.

© 2005 by The National Academy of Sciences of the USA

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